Muscle tissue, like nerve tissue, is electrically active and, when elicited, produces a voltage response which can be recorded.
More particularly, when a motor nerve is electrically stimulated above a certain threshold, an action potential will propagate distally to the muscle innervated by the nerve. The response recorded over that muscle is called a “compound muscle action potential” (or “CMAP”). This measured muscle response is also sometimes referred to as an “M-wave”.
In other words, and looking now at FIG. 1, the CMAP is generated by conduction of the neural impulse from the point of stimulation directly to the innervated muscle (open arrow) and is typically characterized by a latency period (†) and an amplitude (¶).
In addition to a nerve action potential propagating distally towards the muscle (orthodromically), an electrical stimulation will also elicit an action potential which propagates proximally (antidromically) along a motor fiber until it reaches the motor neurons in the spinal cord. There, the potential exists for activated motor neurons to backfire, producing a reflected action potential that propagates orthodromically towards the innervated muscle(s) and causes a second muscle response. The measured muscle response associated with this backfiring is called an “F-wave”. The F-wave is part of a class of neuromuscular responses known as “late responses”.
In other words, and still looking now at FIG. 1, the F-wave response is generated by antidromic action potential propagation along motor nerve axon(s) (shaded arrows) beginning at the point of stimulation, passing through the ventral root(s) of the spinal column to the motor neuron cell body in the spinal cord, backfiring of the motor neurons, and then orthodromic conduction back to the innervated muscle. An individual F-wave response is normally characterized by a number of attributes including latency (‡) and amplitude (*).
In this respect it should be appreciated that F-wave latency is defined as the time difference between the stimulus and the initial deflection of the F-wave response signal from baseline. While other descriptors of F-waves (e.g., amplitude, duration, area, morphology, etc.) are also believed to embody clinically useful information, the F-wave latency (and parameters derived from the F-wave latency) has generally been the most common attribute studied in clinical neurophysiology studies.
Since only a small fraction of the motor neurons backfire, and since the backfiring motor neurons may be different for each stimulus, the F-wave amplitude is, on average, less than 10% that of a CMAP. In addition, the F-wave also occurs much later than the CMAP, since the F-wave is initiated by nerve action potentials that have propagated along a longer path of nerve. F-waves can be recorded with electrodes placed directly on the surface of the skin. When recording electrodes are placed sufficiently adjacent to the activated muscle (such as directly over the muscle motor point), recordings are generally designated “on-muscle”. Recordings can also be made with electrodes place away from the activated muscle. These latter recordings are generally designated “volume-conducted”.
Inasmuch as the backfiring motor neurons are different from stimulus to stimulus, two F-wave recordings will rarely be the same even when identical stimuli and recording electrodes are used. Furthermore, in some instances no neurons will backfire and hence the recorded signal will contain no F-wave response. Thus, for example, in FIG. 2 there is shown a raster of 12 F-wave signals, demonstrating differences between the F-wave responses in different sweeps.
In other words, inasmuch as the probability of any given motor neuron backfiring in response to nerve stimulation is low, the number of impulses propagating back down the nerve, and the resulting F-wave response amplitude, are small and highly variable. In addition, because F-wave responses represent a sampling of the spinal motor neuron pool, sequential F-wave responses tend to differ in latency, amplitude, morphology and, indeed, even whether they are present from one stimulus to the next.
Various neuropathologies can alter both CMAP and F-wave responses. As a result, analyzing CMAP and F-wave responses can be useful in detecting such neuropathology.
More particularly, changes in the amplitude, morphology, or latency of the CMAP generally indicate distal disease, although proximal pathology causing axonal loss can prolong the distal latency and decrease the CMAP amplitude.
The F-wave response reflects conduction along the entire length of the nerve and is thus diagnostically sensitive to nerve root compromise, proximal nerve compression, distal nerve entrapment syndromes, plexopathies, and systemic neuropathies.
More particularly, in a typical nerve conduction study, the nerve is stimulated a number of times and the resulting CMAP and F-wave responses are collected and analyzed. The CMAP generally responds in a highly consistent manner from one stimulus to the next so that an attribute such as latency can be reported as a single value appropriate to all collected signals. By contrast, the attributes of sequential F-wave responses vary. Hence an attribute of the collection of F-wave responses is most appropriately reported as a frequency distribution or as a probability density function. Traditional nerve conduction studies typically report only one F-wave distribution parameter, i.e., the minimum F-wave latency, which is the earliest latency among all recorded F-wave responses. This parameter characterizes conduction of the nerve fibers with the fastest propagation velocities. Other F-wave parameters which have been used include the mean latency among all the F-wave responses, the maximum latency, the median latency, the range of latencies, and the percentage of stimuli evoking detectable F-wave responses (i.e., “persistence”)
F-waves have certain advantages over the CMAP in clinical neurophysiology studies since their associated nerve potentials propagate through a longer stretch of nerve. This is especially true for distinguishing systematic nerve diseases (such as diabetic neuropathy) from localized nerve diseases (such as carpal tunnel syndrome). However, the clinical utilities of F-waves are often limited by the uncertainty of their occurrence, the variability of their morphology and poor signal quality (i.e., noise and baseline drift). This is especially true for “volume-conducted” F-wave recordings, which tend to be significantly smaller in magnitude.
Other neuromuscular response measurements are also known in the art. By way of example but not limitation, such neuromuscular response measurements include A-waves, axon reflex, Hoffman reflex, sensory nerve action potential (or “SNAP”), somatosensory evoked responses, visual evoked responses, auditory evoked responses, etc.
It is believed that neuropathology alters these other neuromuscular response measurements as well. As a result, analyzing these other neuromuscular response measurements can also be useful in detecting such neuropathology.